This invention relates to the field of NMR and MRI and, more particularly, to the field of NMR and MRI where an optical magnetometer is used to detect magnetization.
Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are widely used in healthcare, chemical analysis and geology. In many cases, MRI instruments are large (room-sized), expensive (several million dollars) and cumbersome to operate. The reason MRI instruments are large is that most are based on very high magnetic fields produced by superconducting electromagnets, which require liquid helium cryostats. The high magnetic field is needed for two reasons: 1) to produce a large nuclear polarization within the sample and 2) to generate a large nuclear precession frequency that can then be measured via inductive detection with a coil. Since inductive detection is sensitive to the time derivative of the magnetic flux through the coil, high sensitivity is achieved only at high frequencies (high magnetic field).
Low-field MRI is a growing area within the large MRI field. Remote detection is an important emerging modality for MRI in low magnetic fields. In this technique, the polarization, encoding and detection of nuclei are spatially separated. Polarization is typically accomplished by thermalization in a large magnetic field or using optical pumping techniques. After polarization, the nuclei then flow into the sample to be tested (a human lung, a porous rock, etc.) and DC and RF fields and gradients are applied to “encode” information about the sample onto the nuclear spins. The encoded fluid is then brought into a highly shielded enclosure and the magnetization is recorded as a function of time. By changing the encoding parameters over time, a spatial image of the sample can be created from the time-varying magnetization in the measurement region. Because no especially high magnetic fields are required, the system does not need to be cryogenically cooled, which significantly reduces the complexity and cost.
Low-field MRI places specific requirements on the magnetometers. Perhaps the most important requirement is that the magnetometers operate with high sensitivity at the very low frequencies associated with the low magnetic fields. The most sensitive DC magnetometers have traditionally been superconducting quantum interference devices (SQUIDs) based on Josephson junctions. These have very high sensitivity (approaching 1 fT/Hz1/2) but are again large and cumbersome to operate because of the cryogenic cooling required to reach the superconducting state. Existing compact sensors (magneto-resistive sensors, for example) are typically not sensitive enough to enable low-noise recording of the very weak nuclear polarization.
More generally, remote detection of NMR, in which polarization, encoding or evolution, and detection are spatially separated, has recently attracted considerable attention in the context of MRI (see Xu, S., et al., Magnetic resonance imaging with an optical atomic magnetometer, Proc. Nat. Acad. Sci., 103 129668-129671), microfluidic flow profiling (see Granwehr, J., et al., Dispersion measurements using time-of-flight remote detection MRI, Magn. Res. Imaging, 25, 449-452, and Harel, E., et al., Time of flight flow imaging of two-component flow inside a microfluidic chip, Phys. Rev. Lett. 98, 017601), and spin-labeling (see Anwar, M. S., et al., Spin coherence transfer in chemical transformations monitored by remote NMR, Anal. Chem., 79, 2806-2811). Detection can be performed with SQUIDs inductively at high fields as taught by Granwehr et al., Harel et al., and Anwar et al., or with atomic magnetometer as taught by Xu et al. In order to efficiently detect the flux from the nuclear sample, it is typically necessary to match the physical dimensions of the sensor and the sample. Thus, small, sensitive detectors of magnetic flux reduce the detection volume, thereby reducing the quantity of analyte. Microfabricated atomic magnetometers (see Schwindt, P. D. D., et al., Chip scale atomic magnetometer, App. Phys. Lett., 85, 6409-6411) with sensor dimensions on the order of 1 mm operating in the spin-exchange relaxation free (SERF) regime (see Kominis, I. K., et al., A subfemtotesla multichannel atomic magnetometer, Nature 422, 596-599) have recently demonstrated sensitivities of ˜70 fT/Hz1/2 (see Shah, V., et al., Subpicotesla atomic magnetometery with a microfabricated vapour cell, Nat. Photonics 1, 649-652).